It is common knowledge that in sewing machines having an automatic control for the stitch forming instrumentalities, that is for the transverse oscillations of the needle bar and for the longitudinal displacements of the member feeding the fabric whereon the stitch has to be performed, such controls are given through actuators moved in compliance with given synchronism moments. The actuator controlling the transverse oscillation of the needle bar is moved when the needle comes out of the fabric, whereas the actuator controlling the longitudinal displacement of the fabric feed member is moved when the point of the needle penetrates the fabric. The right moment of initial movement of such actuators is determined by synchronizing signals generated by devices surveying the position of the needle. Therefore such devices generally survey the angular position of sewing machine main shaft, and for example provide two movable magnets fixed to the main shaft and two fixed sensors (i.e. Hall effect sensors) generating the two synchronizing signals for the two actuators, such as accomplished in commonly assinged corresponding U.S. patent applications Ser. No. 41,293, filed May 22, 1979, now patent No. 4,275,624 and entitled "Improved microprocessor controlled electronic sewing machine", and U.S. patent application Ser. No. 973,386, filed Dec. 26, 1978 now U.S. Pat. No. 4,280,424 entitled "Household type sewing machine having microprocessor control", the contents of which are hereby incorporated by reference herein in their entirety. Moreover, in order to attain that when the machine stops the needle is out of the fabric, a third pair of sensors fixed and movable is applied, always connected to the sewing machine main shaft.
Object of the present invention is to realize an improvement to the devices generating synchronizing signals in sewing machines, in such a way that the said devices could be realized in a relatively simple and therefore cheap way, and assuring right working.
Other objects and advantages of the present invention will be apparent from the following description.